Crystal controllable oscillators that are suited to be manufactured in monolithic integrated circuit form and operated as voltage controlled oscillators (VCOs) are well known to those skilled in the art. For example, U.S. Pat. No. 4,571,588 describes such an oscillator. The oscillator circuit disclosed in the '588 patent requires only one additional external pinout from the integrated circuit for connection to an external crystal resonator network. The '558 oscillator includes an oscillation sustaining feedback loop comprising a pair of integrators connected between the input and the output of a differential amplifier and introduces the correct phase shift therebetween to maintain oscillation.
An improved integrated crystal controlled oscillator having a simplified structure to increase manufacturing yields and decrease production costs was described in U.S. Pat. No. 4,646,033. The '033 oscillator has improved frequency stability and the ability to drive emitter-coupled-logic (ECL) loads. The '033 patent is to the same inventor and same assignee as the present patent application. A feature of the '033 oscillator is that a frequency adjusting circuit is connected to the output of a second amplifier. The frequency adjustment circuit introduces variable phase shift within the feedback loop to force the frequency of the crystal to change, thereby permitting the oscillator to be operated as a VCO.
The need exists to permit the switching of crystals via bus in a multiple crystal application, such as a video capture or picture-in-picture (PIP) video system while preserving one external pin per crystal design.